BNETWORK TROUBLESHOOTING METHODOLOGYEXAM OBJECTIVES IN THIS CHAPTERHOW TO USE THE OSI MODEL IN TROUBLESHOOTING 575TROUBLESHOOTING THE PHYSICAL LAYER 581TROUBLESHOOTING THE DATA LINK LAYER 591TROUBLESHOOTING THE NETWORK LAYER 598TROUBLESHOOTING THE TR...

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Network Troubleshooting Methodology

ExAM oBJECTIvES IN ThIS ChApTEr

how To uSE ThE oSI ModEl IN TrouBlEShooTING 575TrouBlEShooTING ThE phySICAl lAyEr 581TrouBlEShooTING ThE dATA lINK lAyEr 591TrouBlEShooTING ThE NETworK lAyEr 598TrouBlEShooTING ThE TrANSporT lAyEr 603TrouBlEShooTING ThE SESSIoN lAyEr 609TrouBlEShooTING ThE prESENTATIoN lAyEr 612TrouBlEShooTING ThE ApplICATIoN lAyEr 614

INTroduCTIoN

Congratulations! You’ve made it almost all the way through the Network+ exam preparation guide. You’ve learned all about the physical and logical components that make up a network, and how to install and configure the Transmission Control Protocol/Internet Protocol (TCP/IP) suite. You’ve also learned about the different protocols that make up the TCP/IP suite as well as how they map to the Open Systems Interconnect (OSI) model. This includes the Internet Protocol (IP), which is the workhorse of the TCP/IP suite that handles the “heavy lifting” of routing TCP/IP traffic from one host to another. You’ve also learned about TCP/IP’s two session layer protocols: TCP, and the User Datagram Protocol (UDP). TCP is a connection-oriented protocol that’s used when each session layer packet has to be acknowledged by the computer it’s being sent to. UDP is connectionless, which is useful for low-overhead connections where speed is at a premium. You’ve also seen the Internet Control Message Protocol (ICMP), which is used for TCP/IP trouble-shooting. In our last chapter, we talked extensively about the various utilities that are available to troubleshoot connectivity problems on a network. In our 573them in a real-world situation to troubleshoot network connectivity issues. We will revisit the OSI model and the TCP/IP-based Department of Defense (DoD) model. We’ll also review the components of the suite of protocols that make up the TCP/IP stack and how common connectivity devices, such as repeaters, bridges, routers, and switches are used to expand or segment TCP/IP networks. All of this is critical information to have at your fingertips when you’re troubleshooting because, just as a physician is better able to treat a sick patient if he knows the person’s background, characteristics, and how the patient normally behaves when he is not ill, you will be at a big advantage when you’re confronted with “sick” or a badly functioning network if you understand your network’s “anatomy” and components. Most networks depend on the TCP/IP protocol to communicate, making it one of the most important “body parts”.The objective of this chapter is to give you a detailed review of TCP/IP, and explain the methodology that will enable you to recognize symptoms of network troubles and to diagnose and correct any errors or misconfigurations that you may find. We all know that a healthy network makes for a happy network administrator, and the information you find in this chapter will help you in preparing both for the Network+ exam and for your journey into the real world of network administration and troubleshooting. To help you with this, we will first go through each layer of the OSI model and talk about the different troubleshooting steps you can take at each layer. For example, the physical layer is concerned with physical connectivity between two computers, so here you’ll look for broken cables or a malfunctioning network interface card (NIC). The data link layer is where switches, bridges, and Ethernet frame types operate, so you’ll troubleshoot this layer by examining all of these components. We will start our review at the physical layer, which is the lowest layer of the OSI model, and work all the way up through the application layer. This chapter will help you to put together all of the concepts you’ve learned throughout this study guide, to help you use them in troubleshooting real-world issues that you might encounter on a network.Exam warning

Be sure that you really understand what is happening at each layer of the OSI model.

It’s important for the Network+ exam that you’re able to recognize the different network

protocols and devices that operate at each level, rather than just memorizing the “Please

Do Not Throw Sausage Pizza Away” or “All People Seem To Need Data Processing”

mnemonics.

how To uSE ThE oSI ModEl IN TrouBlEShooTING

As a network administrator, you should be familiar with the common networking models. In this chapter, we will be using the OSI and DoD models as our guide down the path of troubleshooting. We will learn how to map issues you may encounter back to different levels, which will allow you to pinpoint the devices on which to start troubleshooting. Depending on the layer that you begin your troubleshooting, different techniques may be required to determine the root cause of the issue. For example, if you had a problem with a wireless access point (WAP), which layer would you start with? Why would you select that layer and what could be the issue? What if you had misconfigured your encryption settings for Wireless Encryption Protocol (WEP) and cannot connect to the wireless network? We’ll look at examples like these to see how each layer of the OSI model fits into an overall troubleshooting strategy. Troubleshooting skills are critical in the world of networking and they are also important to learn as you prepare for the Network+ exam.

The purpose of Networking Models

Chapter 6 covered network models and their importance in depth. One of the key benefits of having a network model in place is that it gives you a standard to utilize and reference, thereby helping to isolate network issues to specific layers of a protocol stack. Being able to isolate which layer a problem stems from allows you to identify the types of components that may be involved in the issue, thus helping you to troubleshoot the prob-lem. You’ll often hear the OSI model referenced in everyday networking related conversations, and when used while troubleshooting a problem, it can help everyone involved understand precisely where an issue is occurring. For instance, “We have a Layer 3 problem.” This should immediately bring thoughts of components that exist at Layer 3 of the OSI model such as IP addresses and routers. Because understanding the OSI model is so critical to the network troubleshooting process, it’s imperative that you master the OSI model’s layering.A network protocol is a set of rules used by computers to communicate. Protocols had to be developed so that two computers attempting to transfer data back and forth would be able to understand one another. Some people will describe protocols as “languages,” but this isn’t entirely accurate and can cause confusion, because computer languages are an entirely different concept. A protocol is more like the syntax of the language, which refers to the order in which the words are put together, rather than the language itself.